1. Field of the Invention
The present invention is directed to underwater accumulator systems which provide pressurized working fluid.
2. Description of Related Art
Deepwater accumulators provide pressurized working fluid for the control and operation of equipment, e.g. for blowout preventer operators; gate valves for the control of flow of oil or gas to the surface or to other subsea locations; hydraulically actuated connectors; and similar devices. The fluid to be pressurized is typically an oil based product or a water based product with added lubricity and corrosion protection, e.g., but not limited to hydraulic fluid.
Certain prior art accumulators are precharged with pressurized gas to a pressure at or slightly below an anticipated minimum pressure required to operate equipment. Fluid can be added to the accumulator, increasing the pressure of the pressurized gas and the fluid. The fluid introduced into the accumulator is stored at a pressure at least as high as the precharge pressure and is available for doing hydraulic work.
Such prior art accumulators include: a bladder type with a bladder to separate the gas from the fluid; a piston type having a piston sliding up and down a seal bore to separate the fluid from the gas; and a float type with a float providing a partial separation of the fluid from the gas and for closing a valve when the float approaches the bottom to prevent the escape of gas.
In one particular example, a prior art system has accumulators that provide typical 3000 psi working fluid to surface equipment has a 5000 psi working pressure and contain fluid which raises the precharge pressure from 3000 psi to 5000 psi The efficiency of accumulators is decreased in deepwater; e.g., 1000 feet of seawater the ambient pressure is approximately 465 psi and, for an accumulator to provide a 3000 psi differential at 1000 ft. depth, it is precharged to 3000 psi plus 455 psi, or 3465 psi At slightly over 4000 ft. water depth, the ambient pressure is almost 2000 psi, so the precharge is required to be 3000 psi plus 2000 psi, or 5000 psi, i.e., the precharge equals the working pressure of the accumulator. Any fluid introduced for storage causes the pressure to exceed the working pressure, rendering the accumulator non-functional.
In the deepwater use of accumulators the ambient temperature can decrease to about 35 degrees F. For an accumulator precharged to 5000 psi at a surface temperature of 80 degrees F., about 416 psi precharge is lost simply because the temperature was reduced to 35 degrees F. The rapid discharge of fluids from accumulators and the associated rapid expansion of the pressurizing gas causes a natural cooling of the gas so that an accumulator is quickly reduced in pressure from, e.g., 5000 psi to 3000 psi without heat coming into the accumulator (adiabatic), experiences a pressure drop to 2012 psi
U.S. Pat. Nos. 7,108,006; 6,202,753; 4,777,800; 4,649,704; and 3,677,001 are illustrative of various prior art systems and are mentioned here not by way of limitation nor as exhaustive of the available prior art; and all said patents are incorporated fully herein for all purposes. FIG. 1 illustrates a system as disclosed in U.S. Pat. No. 3,677,001 which shows a submerged pipeline 10 on which is arranged a valve housing 11 which contains a valve member to open and close pipeline 10 to control the flow of fluid therethrough. A valve stem housing is mounted on valve housing 11. A valve stem 13 extends through the valve stem housing and connects to a piston 14 arranged in an actuator cylinder 15. Piston 14 has fixed power and exhaust strokes. The valve stem housing is provided with packing seals 17 which surround and seal off fluid flow around valve stem 13. A reduced internal diameter portion 20 of actuator cylinder 15 forms a cavity or chamber 21 and a seating shoulder 23. A mating shoulder 22 formed on piston 14 is adapted to engage shoulder 23. A static seal 24 which suitably may be an “O”-ring is arranged in a recess in shoulder 23 and seals off the space between shoulders 22 and 23 when piston 14 is at the end of its power stroke, as shown in the figure. A spring 25 is arranged in chamber 21 and functions to move piston 14 in its exhaust stroke. When the valve is fully open, piston 14 is at the end of its power stroke and when the valve is fully closed the piston is at the end of its exhaust stroke. When the valve (or other equipment) to be operated is located at a remote offshore location, a hydraulic power fluid reservoir 30 is provided with a floating piston 31, compensated by sea water pressure. A diaphragm could be substituted for piston 31 A conduit 34 supplies a pump 32 with hydraulic control fluid from reservoir 30. Pump 32 is operated by electrical power supplied from the water's surface through a conductor 33. An accumulator 35 is connected to pump 32 to the exhaust stroke end of actuator cylinder 15 by means of a conduit 40. The purpose of the accumulator is to provide a supply of power fluid available for immediate delivery to cylinder 15. A bypass conduit 41 connects conduit 40 to reservoir 30. A solenoid operated valve 45 controlled by electrical power supplied from the water's surface through a conductor 46 is connected into conduit 41. Another solenoid operated valve 47 supplied with operating power from the water's surface through a conduit 48 is arranged between accumulator 35 and the junction of conduits 40 and 41. An additional conduit 50 connects chamber 21 to reservoir 30.
There has long been a need, recognized by the present inventor, for an effective accumulator systems and pressure compensation systems for underwater and subsea use. There has long been a need, recognized by the present inventor, for such systems which increase the amount of available pressurized gas to enhance the operation of subsea working fluid systems.